The present invention provides a gallium nitride (GaN) structure and resulting devices. Gallium nitride is a semiconductor compound of interest because its direct bandgap provides it with the potential to emit blue light with high efficiency.
The blue portion of the visible spectrum and the adjacent ultraviolet (UV) wavelengths (approximately 500-350 nanometers; 2.5-3.16 eV) are considered important regions of the entire electromagnetic spectrum. In spite of the interest in producing semiconductor devices that emit blue, violet or ultraviolet wavelengths (or corresponding detector devices), efforts to develop such devices has been difficult at best and unsuccessful in most cases. Presently, many light-emitting diodes ("LEDs," also referred to as "semiconductor optical devices") are available which can produce light or electromagnetic radiation from the infrared to the green wavelengths (approximately 1000-500 nanometers; 1.2-2.5 eV). As recognized by anyone familiar with the production of color and color images, however, blue light is also required as the third primary color of the visible spectrum in order to give complete full color imaging and graphics in appropriate applications.
Gallium nitride (GaN) is an interesting candidate for blue light-emitting diodes because of its relatively direct wide bandgap. As is known to those familiar with semiconductor materials and devices, and the interaction of semiconductor materials with light, the color of light as seen by the human eye represents a wavelength (or its corresponding frequency). In turn, the wavelength and frequency correspond to a given energy value. Thus, particular colors of the visible spectrum can only be produced by materials in which energy transitions of the required amount can take place. Stated most simply, the color that can be produced by a light-emitting diode of a given semiconductor material is a direct function of the material's bandgap. Wider bandgaps permit greater energy transitions which in turn produce higher energy photons leading to higher frequency (lower wavelength) colors (frequency being directly proportional to the energy transition and wavelength being inversely proportional to frequency).
Gallium nitride has a sufficient bandgap (3.4 eV) to emit any color in the visible spectrum, and particularly blue light, but suffers from certain fundamental difficulties. One difficulty in using gallium nitride for semiconductor light-emitting diodes is the difficulty in identifying a suitable substrate material. As is well known to those familiar with such devices, LEDs emit light when current (a flow of electrons) passes across a junction between p-type and n-type layers of a semiconductor material. When electrons and electron vacancies (holes) recombine, photons are emitted of a wavelength corresponding in some fashion to the material's bandgap as set forth above. Generally speaking, such layers, however, must be characterized as single crystal ("epitaxial") layers. This in turn requires that they be grown on a suitable substrate. Therefore, as is known to those familiar with crystal growth, the substrate will greatly influence the epitaxial growth mechanism and quality that will take place upon it. In general, in order for a desired type of epitaxial growth to take place, the crystal lattice parameters of the substrate and those of the epitaxial layer must be either identical or reasonably close to ,one another. A crystal layer may grow on a non-matching substrate, but it will grow in an amorphous form or full of defects, either of which will essentially destroy its useful electronic or electro-optical properties. It is well established that the crystal structures of epitaxial layers of gallium nitride are very strongly influenced by the substrate material and its orientation.
Identifying a suitable substrate for gallium nitride has been a difficult task for researchers as gallium nitride bulk substrates have never been successfully produced. Sapphire (Al.sub.2 O.sub.3) has been the primary substrate choice. To date, sapphire has provided a somewhat useful, although less than ideal, thermal and crystal match for gallium nitride.
Sapphire has one glaring disadvantage: its lack of conductivity; i.e., sapphire is very difficult to dope to produce a conductive substrate. In device manufacture, if a substrate lacks conductivity, then all of the electrical contacts to the device (usually two contacts for LEDs) must be made other than to the substrate. As a result, gallium nitride devices formed on sapphire substrates typically require that at least two electrical leads be placed on the same surface of the device.
A different LED structure is often preferred, however, which is referred to as a "vertical" structure, and one requiring that the LED be formed on a conductive substrate. In vertical LEDs, ,electrical contacts can be made to the top and bottom of the device rather than both two on the top, or one or both on the side, all of which arrangements are usually more difficult to engineer than is the top and bottom contact orientation of a typical LED.
One proposed candidate material for a conductive substrate for gallium nitride, and thus for vertical GaN LEDs, is silicon carbide (SIC). Silicon carbide has an appropriate thermal match with gallium nitride, i.e., the coefficients of linear expansion for both materials are quite similar. Silicon carbide can be conductively doped, and indeed is a blue light-emitter in its own right; see e.g., Edmond, et al. U.S. Pat. Nos. 4,918,497 and 5,027,168, both of which are assigned to the assignee of the present invention.
Silicon carbide has a better, but not identical lattice match with gallium nitride, however, making high quality epitaxial layers of GaN difficult to produce directly on SiC. SIC's lattice match is closer to GaN, however, than is sapphire's (sapphire has a 15% lattice mismatch with GaN, SiC has 3.5% mismatch). Thus, successful vertical devices of gallium nitride on silicon carbide have yet to appear in commercial quantities.
A number of the characteristics of gallium nitride, the difficulties in working with it, and attempted solutions to these problems are set forth by Strite and Morkoc, GaN, AlN, and InN: A Review, J. Vac. Sci. Technol. B., 10(4), July/August 1992, pp. 1237-1266.